Silicon nitride (Si3N4) is an excellent material in terms of strength, toughness, corrosion resistance, oxidation resistance and thermal shock resistance, and is thus widely used in cutting tools, gas turbines, bearings and so on. Using silicon nitride for a structural material such as engine components has recently been researched, and more wear resistance, hardness and so on are required more severely for silicon nitride in their level of performance.
For example, in the case that a silicon nitride-based composite material is used in tools for deformation or specific automobile components for which high wear resistance is required, a markedly higher wear resistance is required than with a conventional material such as a hard metal (a cermet material comprising hard particles made of WC and a binding phase of Co or the like) or high-speed steel.
However, silicon nitride-based composite materials are more expensive than the above materials, and the current situation is that the wear resistance is not at a satisfactory level commensurate with the cost.
Note that ‘silicon nitride-based’ refers to ceramics containing silicon nitride (Si3N4) and/or sialon as a main crystalline phase. Moreover, ‘silicon nitride-based composite material’ refers to a material comprising a matrix having a silicon nitride-based ceramic as main crystals thereof, and a different component dispersed and composited in the matrix.
Various researches have been carried out to further improve the properties of such silicon nitride-based materials. For example, in Japanese Patent Publications No. 11-139882 and No. 11-139874, it was reported that by mixing together a silicon nitride powder and a metallic titanium powder at high acceleration in a nitrogen atmosphere, a composite powder comprising fine silicon nitride particles and titanium nitride particles can be obtained. Furthermore, it was also reported that by using this composite powder, a silicon nitride sintered body having a fine crystalline structure and a high strength can be produced because the titanium nitride particles suppress the grain growth of the silicon nitride.
Although the above silicon nitride sintered body exhibits a high strength, research had still not been proceeded into the properties relating to friction as a material for machine structural use, in particular into reducing the friction under lubricant-free conditions, which is most promising with regard to the current trend towards energy-saving.
Moving on, as a common method to produce a ceramic material having a low friction coefficient, a method in which a solid lubricant such as boron nitride, molybdenum sulfide or graphite is dispersed in the material is well known. However, regarding the second phase of the solid lubricant, dispersion is only possible down to about submicron size, and hence there has been a limit to how much the friction coefficient can be reduced.
Moreover, in Japanese Patent Publication No. 11-43372, a silicon nitride-based ceramic was proposed and the ceramic contains 0.5 to 50 wt % of free carbon, for which the mean minor axis diameter of the silicon nitride-based crystal grains is 0.5 μm or less, and the ceramic also has a friction coefficient under lubricant-free conditions of 0.2 or less. However, with such a combination of silicon nitride and free carbon, the comparative wear amount is low at 10−7 mm2/N, and hence problems still remain with regard to the wear amount.